SEISMIC VULNERABILITY MANUALof bridges as to the extent of their vulnerability to these failure modes. The procedure that follows clearly outlines the NYSDOT approach to the seismic

SEISMIC VULNERABILITY MANUAL

NEW YORK STATEDEPARTMENT OF TRANSPORTATION

STRUCTURES DESIGN AND CONSTRUCTION DIVISIONBRIDGE SAFETY ASSURANCE UNIT

OCTOBER 1995REPRINTED AUGUST 1998REVISED NOVEMBER 2002REVISED NOVEMBER 2004

Copyright © by New York State Department of Transportation

All rights reserved. No part of this publication may be stored in a retrieval system, transmitted, or reproduced in anyway, including but not limited to photocopy, photograph, magnetic or other record, without the prior agreement andwritten permission of the publisher.

i

FOREWORD

The majority of catastrophic bridge failures around the world have occurred for reasons otherthan those that are primarily condition-based. The collapse of the New York State ThruwayAuthority’s Schoharie Creek Bridge during heavy flooding in April, 1987 is one such example. In order to eliminate or reduce the vulnerability of new and existing bridges to such catastrophicfailures, the New York State Department of Transportation (NYSDOT) initiated acomprehensive Bridge Safety Assurance (BSA) Program. This program consists of a multi-stepprocess for identifying potential causes, or modes, of bridge failure and for the subsequent ratingof bridges as to the extent of their vulnerability to these failure modes. The procedure thatfollows clearly outlines the NYSDOT approach to the seismic vulnerability failure mode as itrelates to new bridges, existing bridges and bridges programmed for rehabilitation.

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ii

TABLE OF CONTENTSPage

SECTION 1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1

1.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.11.2 Seismic Retrofit Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1

1.2.1 Vulnerability Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.11.2.2 Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.21.2.3 Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4

1.3 Manual Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4

SECTION 2 SCREENING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1

2.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.12.2 Inventory Screening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.12.3 Susceptibility Grouping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2

2.3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.22.3.2 Group Assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2

SECTION 3 CLASSIFYING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1

3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.13.2 Overview of Classification Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.13.3 Calculation of Classification Scores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2

3.3.1 Vulnerability Score (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.23.3.1.1 Vulnerability Score for Connections, Bearings and

Seatwidths, V1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.43.3.1.2 Vulnerability Score for Columns, Abutments, and

Liquefaction Potential, V2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.103.3.2 Seismic Hazard Score (E) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.14

3.4 Assignment of Seismic Vulnerability Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.16

SECTION 4 RATING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1

4.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.14.2 Rating Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1

4.2.1 Likelihood of a Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.24.2.2 Consequence of Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2

SECTION 5 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1

APPENDIX A FUNCTIONAL IMPORTANCE (BRIDGE CRITICALITY) . . . . . . . . . . . . A.1APPENDIX B NYSDOT SEISMIC PERFORMANCE CATEGORIES . . . . . . . . . . . . . . . . B.1APPENDIX C VULNERABILITY RATING SCALE . . . . . . . . . . . . . . . . . . . . . . . . . . . . C.1APPENDIX D NYSDOT MINIMUM REQUIRED SEAT WIDTH . . . . . . . . . . . . . . . . . . . D.1

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LIST OF TABLESPage

Table 3.1 Bearing Transverse Restraint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6Table 3.2 Values of R . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.12Table 3.3 Potential for Liquefaction-related Damage . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.14Table 3.4 Site Coefficient, S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.15Table 3.5 Alternative Site Coefficients, S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.16Table 3.6 Seismic Vulnerability Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.16Table 4.1 Vulnerability Rating descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1Table 4.2 Vulnerability Rating Score Ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2Table 4.3 Likelihood of Failure Scores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3Table 4.4 Failure Type Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4Table 4.5 Failure Type Rating Scores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5Table 4.6 Exposure Rating Scores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6

LIST OF FIGURES

Figure 1.1 Seismic Vulnerability Assessment Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3Figure 1.2 Seismic Vulnerability and Retrofit Program . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5Figure 2.1 Inventory Screening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3Figure 2.2 Susceptibility Grouping Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5Figure 3.1 Assignment of Bridge Vulnerability Score (V) . . . . . . . . . . . . . . . . . . . . . . . . . 3.3Figure 3.2 Seismically Vulnerable Bearings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5Figure 3.4a Calculation of Bearing Vulnerability Score (V1) . . . . . . . . . . . . . . . . . . . . . . . . 3.7Figure 3.4b Calculation of Bearing Vulnerability Score (V1) Cont’d . . . . . . . . . . . . . . . . . . 3.8Figure 4.1 Vulnerability Rating Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1Figure 4.2 Vulnerability Rating Summary Sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6

SECTION 1 - INTRODUCTIONPage 1.1

SECTION 1 - INTRODUCTION

1.1 Purpose - The purpose of this document is to describe the NYSDOT procedure forassessing and rating the seismic vulnerability of the bridges in New York State. Thisprocedure is part of the NYSDOT seismic evaluation and retrofit program for highwaybridges which is intended to reduce the vulnerability of the state's bridges to failures causedby earthquakes.

The NYSDOT seismic vulnerability assessment is a series of screening and classificationsteps which result in a vulnerability rating for each bridge. This rating describes thelikelihood and the consequence of a failure in terms of the urgency in which correctiveactions need to be implemented.

The vulnerability ratings developed under this program are designed to be used inconjunction with similar ratings for other extreme events, such as scour and overload, inorder to establish priorities for taking corrective actions on any given bridge.

1.2 Seismic Retrofit Process - The process of seismic retrofitting an inventory of bridgesinvolves the assessment of a number of complex issues and requires considerableprofessional judgement. An appreciation for the economic, social and technical issues isimportant to the successful execution of such a program. It is therefore helpful to divide theprocess into three major stages. These are as follows:

! Vulnerability assessment of the bridge inventory! Detailed evaluation of deficient bridges! Implementation of retrofit measures

Figure 1.2 shows these three stages and their interrelation with each other. A briefdescription of each is given in the following sections.

1.2.1 Vulnerability Assessment - The seismic assessment of a large inventory of bridges isintended to identify those bridges that are seismically deficient and establish an order ofpriorities for taking corrective actions, i.e. retrofit or replacement. Criteria used forestablishing this order are based on degree of vulnerability, and the likelihood andconsequences of failure.

The process is comprised of three steps: screening, classifying and rating as illustrated inFigure 1.1. It is intended that these steps be performed sequentially and that once thescreening step is complete and a preliminary set of priorities is determined, the remaining two steps are applied in order of these priorities. This will result in a staggeredprogression of bridges through the assessment process.

SEISMIC VULNERABILITY MANUAL Page 1.2

Step 1: Screening - The purpose of this step is to develop a preliminary ranking ofbridges in the inventory, using information in the data base of the Bridge Inventory andInspection System (BIIS) [1]. Using such factors as date of construction, seismicacceleration coefficient, importance, span configuration, bearing details, and type of pierand foundation, bridges are assigned to one of four susceptibility groups. This groupingis, in effect, a preliminary, 4-level, ranking of the inventory from greatest to leastvulnerability. This ranking is later used to determine the order in which bridges areprogressed to the classifying step where more detailed assessments are carried out.Details of Step 1: Screening, are given in Section 2 of this Manual.

Step 2: Classifying - The purpose of this step is to evaluate in greater detail the seismicvulnerability of each bridge identified in Step 1 as potentially having inadequate seismicload capacity or details. The order in which this assessment is done is also determined bythe results of Step 1. Access to BIN folders, as-built plans and inspection reports willgenerally be necessary to complete this step, along with one or more site visits to confirminformation and obtain additional data which may be missing from the BIN folder. Theproduct of this exercise is a classification score which quantifies the potentialvulnerability of each bridge relative to other bridges in the inventory. It is also used toassign a seismic vulnerability class (high, medium or low) to each bridge which is laterused to obtain a vulnerability rating score in the next and final step in this procedure. Themethodology used to develop the classification score is largely based onrecommendations prepared by the Federal Highway Administration (FHWA) and ispublished in the Seismic Retrofitting Manual for Highway Bridges [2]. Details of Step 2:Classifying are given in Section 3 of this Manual.

Step 3: Vulnerability Rating - The purpose of this step is to provide a uniform measureof a structure's vulnerability to failure on the basis of its seismic vulnerability class andthe consequences of failure. The resulting seismic vulnerability rating is compatiblewith similar ratings for other Bridge Safety Assurance (BSA) failure modes and indicatesthe need for, and the urgency by which, corrective actions should be taken. The rating iscalculated by first assigning a likelihood of failure score (using the vulnerability class)to the bridge and then adding to it a consequence of failure score. This latter score isbased on an estimation of the failure type and an exposure score which are calculated inaccordance with standard BSA procedures[3]. Details of Step 3: Vulnerability Rating aregiven in Section 4 of this Manual.

1.2.2 Evaluation - A Structural Integrity Evaluation (SIE) should be carried out before anycorrective actions are taken on bridges that have been identified during the VulnerabilityAssessment stage (Section 1.2.1) as seismically inadequate. This evaluation, as defined inthe NYSDOT Uniform Code of Bridge Inspection [7], includes a detailed analysis of allof a bridge’s vulnerability modes, including seismic. The purpose of the SIE is twofold. First, the more detailed seismic analysis will define which component(s) of the structureis seismically vulnerable and quantify its inadequacy. Second, it will be used to designand determine the benefit of any proposed counter measures. An S.I.E. is also requiredbecause the vulnerability assessment procedures used in Section 1.2.1 are generallyover-conservative in order to assure that, as far as possible, all deficient bridges areidentified.

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FIGURE 1.1 - Seismic Vulnerability Assessment Procedure


Methods for performing this evaluation generally fall into two categories as follows:

! Capacity/Demand (C/D) ratio methods! Push-over methods (lateral strength methods of analysis)

C/D methods evaluate each component in a bridge on a member-by-member basis andcomponents with ratios less than unity are identified for corrective action. The method isrelatively straightforward to apply and is similar to load rating a bridge for live loads. Itis generally conservative and sometimes overly-conservative because it ignores theinteraction between the various components of a bridge and the load redistribution thatoccurs during an extreme event such as an earthquake. Push-over methods address thisissue but are more time consuming to apply. Nevertheless this extra effort may be offsetby a reduction in the extent and cost of the required corrective actions. Both methods aredescribed in Chapter 3 of the FHWA Retrofitting Manual [2].

1.2.3 Implementation - The design and implementation of appropriate retrofit measurescomprise the final stage in the seismic retrofitting process. Corrective measures havebeen developed for most of the common deficiencies found in highway bridges. Theseare based on experience with past earthquakes and extensive research and developmentsponsored by Caltrans and FHWA. They include measures for bearings, seats andexpansion joints; columns, cap beams and structural joints; and foundations and siteswith poor soil conditions. These options are described in Chapters 4 through 9 of theFHWA Retrofitting Manual [2] as well as other publications such as Caltrans SeismicDesign References [4].

1.3 Manual Outline - This manual describes the assessment procedure to be used todetermine the seismic vulnerability rating for each bridge in the New York State BridgeInventory. An outline of this 3-step procedure is given above in Section 1.2.1. where it isshown that this procedure involves screening, classifying and rating processes. Each ofthese steps is described in detail in manual Sections 2, 3 and 4, respectively. However,this document does not discuss the detailed evaluation of inadequate bridges or thedesign and implementation of retrofit measures, since both topics are well described inthe literature such as the FHWA Seismic Retrofitting Manual for Highway Bridges [2].Evaluation procedures and retrofit strategies in these publications are generic by design and should be applied to New York State bridges without difficulty. The FHWA Manualmay be used as a guide; however, it must be emphasized that the minimum designrequirements originally issued under NYSDOT Engineering Instruction 92-46 andincorporated into the New York State Standard Specifications for Highway Bridges [8],shall always be satisfied when retrofitting existing bridges for earthquake forces.

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VULNERABILITY ASSESSMENT

SCREENCLASSIFY

RATE

EVALUATION

IMPLEMENTATION

Figure 1.2 - Seismic Vulnerability Assessment and Retrofit Program


SECTION 2 - SCREENING

2.1 General - Screening is the first step in the Vulnerability Assessment program and itspurpose is to evaluate a large population of bridges in an efficient manner in order todevelop a preliminary ranking of bridge vulnerability. Using only information which is inthe data base of the Bridge Inventory and Inspection System (BIIS), bridges are assignedto one of four susceptibility groups according to their assessed vulnerability. No analysisis conducted during this screening. If necessary, refinements to these assessments aremade in Section 3: Classifying and again during a Structural Integrity Evaluation (SeeSection 1.2.2).

Information from BIIS that is used to perform this screening includes:

! Date of construction! Importance: critical facility, utilities carried, AADT, bypass length, function

classification! Single or multiple spans! Simple or continuous girders! Bearing type! Number of girders per span (girder redundancy)! Skew! Pier type! Footing type

The screening process described below is in two parts: first a preliminary screening toidentify those bridges that should be assessed and second the assignment of these bridgesto susceptibility groups. These two parts are separately described in the followingsections.

2.2 Inventory Screening - The BIIS is a comprehensive data base of highway structures ofvarious types and the first step is to exclude those structures that are either not bridges orbridges deserving special study. The following questions is asked: Is the structure aspecial type?

! Tunnel or culvert? (Yes: exclude and assign rating of 6). Tunnels and culvertshave historically performed very well under seismic loads.

! Arch, suspension or stayed girder? (Yes: perform SIE)! Moveable bridge? (Yes: perform SIE)! Railroad or pipeline? (Yes: if over a highway, perform SIE; if not, assign rating

of 6)! Temporary or closed? (Yes: if over a highway, perform SIE; if not, assign rating

of 6)! Long span > 500 feet? (Yes: perform SIE)

The bridge types listed above as needing an SIE should be given an informalclassification of high, medium or low, based on engineering judgement and thendetermine a rating using the rating procedure (See Sections 3 and 4). Using thedefinitions of vulnerability ratings, (See Appendix C) the Evaluator will have some

SECTION 2 - SCREENINGPage 2.2

guidance on when the Structural Integrity Evaluation should be done. This process isillustrated in Figure 2.1.

2.3 Susceptibility Grouping

2.3.1 General - Once the bridge inventory has been screened as indicated in Section 2.2, theassignment to susceptibility groups can be made. Four groups are defined as follows:

Susceptibility Group 1: High seismic vulnerabilitySusceptibility Group 2: Moderate-high vulnerabilitySusceptibility Group 3: Moderate-low vulnerabilitySusceptibility Group 4: Low seismic vulnerability

Assignment to one of these four groups is based on the eight structural parameters listedin Section 2.1. This process is shown in Figure 2.2 and described in the next section.

2.3.2 Group Assignments - As shown in Figure 2.2, there are six basic steps to the assignmentprocess and a number of intermediate steps as described below:

Step A: If the bridge is a single span bridge, its vulnerability is limited to bearingsand connections at the abutments as described in Steps B and C. If thebridge has multiple spans connectivity, pier and foundation types affectvulnerability, as described in Steps D, E and F below.

Step B: If the bridge has integral abutments it is assigned to Group 4.

Step C1: If the abutment bearings are steel rocker bearings, which have a tendency tooverturn during large displacements, the bridge is assigned to Group 2.

C2: If the abutment skew is greater than 30o, the bridge is assigned to Group 2regardless of bearing type; for smaller skew angles, Group 4 is assigned.

Step D: If the bridge consists of multiple spans, seismic vulnerability is stronglyinfluenced by the connectivity at the piers. This is because continuous girdersare inherently more stable than simple spans which are particularlyvulnerable to unseating modes of collapse. Continuous girder bridges areexamined during Steps E1 through E7 below.

Simply supported spans are considered in Steps F1, F2 and F3 where bearing type, skewand redundancy are checked. Even if all responses are negative, the bridge is assigned toGroup 2. There is no need to check pier and footing conditions at this time, since thesewill be examined when Group 2 bridges are Classified (Section 3).


Is Structure Tunnel or Culvert ?

Is Structure Arch, Cable - Stayedor Suspension Bridge ?

Is Structure Moveable Bridge?

Is Structure Railroad or PipelineBridge over Highway ?

Is Structure Temporary or Closed Bridge over Highway ?

Is Structure Railroad or PipelineBridge not over Highway ?

Is Structure Temporary or ClosedBridge not over Highway ?

Does Structure have one or moreLong Spans ( > 500 ft. ) ?

Susceptibility GroupingExclude and Rate as 6 Structural Integrity Evalution (SIE)

Yes No

Yes No

Yes

Yes

No

No

No Yes

Yes No

No

Yes No

Yes

Figure 2.1. Inventory Screening


Step E1: If the continuous girder is supported on steel rocker bearings, the bridge isassigned to Group 2 (see also Steps C1 and F1).

E2: If the skew is greater than 30o and/or bridge on curved alignment, the bridgeis assigned to Group 2 regardless of bearing type.

E3: If the continuous superstructure comprises only 2- or 3-girders or trusses, ithas poor redundancy and little resistance to collapse if lateral restraint is lostat an edge girder bearing; the bridge is assigned to Group 2 regardless ofbearing type.

E4: If the piers are unreinforced (solid concrete or solid stone), the bridge isassigned to Group 2.

E5: If each pier is a single column, the bridge is assigned to Group 3.

E6: If the piers are timber or steel pile bents, or a timber trestle bent, the bridge isassigned to Group 3.

E7: If the footings are concrete and supported on piles or earth, the bridge isassigned to Group 3. Concrete footings on rock are assigned to Group 4. Ifthere are no affirmative response in Steps E1 through E7, Group 4 isassigned.

Step F1: If the simply supported girders are supported on steel rocker bearings, thebridge is assigned to Group 1 (see also Steps C1 and E1).

F2: If the skew is greater than 30o and/or bridge on curved alignment, the bridgeis assigned to Group 1 regardless of bearing type.

F3: If the superstructure comprises only 2- or 3-simple girders or trusses, it haspoor redundancy and little resistance to collapse if lateral restraint is lost atan edge girder bearing; the bridge is assigned to Group 1 regardless ofbearing type. If there are no affirmative responses in Steps F1 through F3,Group 2 is assigned.

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Inventory Screening

A: Single Span?

E7: Piles or SpreadFootings on earth ?

D: Continuous Girder ?

E1: Rocker Bearings ?

E2: Skew > 30 ?

E3: 2 or 3 Girders or Trussses ?

E4: Unreinforced Piers ?

E5: Single Column Piers ?

E6: Steel Pile Bents ?

GROUP 4GROUP 3

B: Integral Abutments?

C1: Rocker Bearings ?

C2: Skew > 30 ?

GROUP 4GROUP 2

F1: Rocker Bearings?

F2: Skew > 30 ?

F3: 2 or 3 Girdersor Trusses ?

GROUP 2GROUP 1

yes

yes

yes

no

no

no

no

no

no

no

yes

yes

no

yes

no

no

no

yes

no

yes

yes

yes

yes

yes

yes

yes

yes

Figure 2.2 - Susceptibility Grouping

SECTION 3 - CLASSIFICATIONPage 3.1

SECTION 3 - CLASSIFYING

3.1 General - The purpose of the classifying step is to assess the vulnerability of a structureto seismic damage. The product of this step is a classification score which serves twopurposes: first, it quantifies the potential vulnerability of a bridge to seismic damagerelative to other bridges in the classifying process and second, the classification score isused to place a bridge into a high, medium or low seismic vulnerability class.

In addition to the above information, it is also necessary to know the design seismicacceleration coefficient (A) for each bridge site. These coefficients should represent theeffective peak acceleration at a site for an earthquake that has a 10% probability of beingexceeded in any 50 year period (a return period of approximately 475 years). A map giving such a set of coefficients is contained in Division I-A of the AASHTO StandardSpecifications for Highway Bridges [5]. Based upon this map, the NYSDOT StandardSpecifications for Highway Bridges [8] divides New York State into three SeismicPerformance Categories: Seismic Performance Category ‘A’, Seismic PerformanceCategory ‘B’ and the New York City (Downstate) Area. The limits of these Categoriesare shown in Figure 6A.2-2 of the NYSDOT Standard Specifications for HighwayBridges and reproduced in Appendix ‘B’ of this manual.

The classification procedure requires additional information on other parameters whichstrongly influence seismic performance; including soil type, attached seat widths atexpansion joints, and pier reinforcement details. The additional data may be found as aresult of a site visit or from information on as-built plans. Where required data isunavailable, some conservative assumptions may be made to complete the classificationprocess.

The procedures in the classifying process have been adapted from the FHWA SeismicRetrofitting Manual for Highway Bridges [2], and have been designed to provide adegree of uniformity between the results of different evaluating engineers and to ensurethat all the factors which affect seismic performance are considered. The procedures arenot intended to exclude the judgement of a qualified professional trained in earthquakeengineering.

3.2 Overview of Classification Process - Except as noted below, all bridges that have beenassigned to susceptibility groups in Section 2.3 are to be classified in accordance with theprocedures given in this and subsequent sections. Bridges exempt from this classificationprocess are as follows:

1. If the date of construction is later than 1990, and the bridge, includingsubstructures, has been constructed to NYSDOT Standards as evidenced bycontract documents, it can be assumed to have adequate seismic resistance. Noremedial actions are required and a rating of 5 is assigned in Section 4 (Table 4.1).

2. If the bridge is located in Seismic Performance Category (SPC) A and it is not acritical facility then seismic assessment is not required and no action is required. A rating of 5 is assigned if the structure is designed to current seismic standards

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and the structure is rated a 4 if not designed to current seismic standards, asdescribed in Section 4 (Table 4.1). The reason for excluding these bridges frompossible retrofit actions regardless of age or vulnerability, is the very lowlikelihood of a damaging earthquake occurring during the remaining useful life ofthe bridge. On the other hand if the bridge is a critical facility, it must be screenedfor potential vulnerabilities despite the low level of seismic hazard. A procedurefor identifying critical bridges is given in Appendix A.

A classification score (CS) is calculated for all non-exempt bridges as:

CS = V A E (3.1)

where V is a numerical measure of structural vulnerability and E is a seismic hazardrating for the site.

Both V and E are assigned values which can range from 0 to 10 and the value for theclassification score (CS) can therefore range from 0 to 100. A low value for CS implies alow seismic risk and a high value for CS implies a high risk. These values for CS(together with engineering judgement) are used to assign seismic vulnerability classesas described in Section 3.4.

3.3 Calculation of Classification Scores - As noted above, the classification score is afunction of structure vulnerability (V) and seismic hazard (E). Calculation of V and E isdescribed in the following sections. A field evaluation of the bridge will be necessary tocomplete these calculations. These field visits will be used to confirm inventory data andto obtain additional information used in the assessment procedure.

3.3.1 Vulnerability Score (V) - Although the performance of a bridge is based on theinteraction of all its components, it has been observed in past earthquakes that certainbridge components are more vulnerable to damage than others. These are: (a)connection bearings and seats, (b) piers, (c) abutments, and (d) soils. Of these, bridgebearings seem to be the most economical to retrofit. For this reason, the vulnerabilityscore to be used in the classification process is determined by examining the connections,bearings and seat details separately from the remainder of the structure. Connectionsrefer to whether the superstructure is continuous or interrupted by joints. A separate vulnerability score V1, is calculated for these components. The vulnerability score for theremainder of the structure, V2, is determined from the sum of the vulnerability scores foreach of the other components (piers, abutments and soils) which are susceptible tofailure. The overall score for the bridge is then selected from V1 and V2 according toseverity of the seismic hazard (Seismic Performance Category, SPC)* and the importance(criticality)** of the bridge as follows:

*Seismic Performance Categories (SPC) are defined for each New York State County inAppendix ‘B’.**Note: A critical bridge is defined in Appendix A (see also Step 2 of the screening procedure(Section 2.2)).

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Calculate VulnerabilityScore for Connections,

Bearings and Seat Widths, V1

Calculate PierVulnerability Score, PV

Calculate Abutment Vulnerability Score, AV

Calculate LiquefactionVulnerability Score, LV

V2 = PV + AV + LV< 10

V =max ( V1 , V2 )

V = V1

V =max ( V1 , V2 )

V = V1

V = V1

N / A

CriticalBridge

Non-critical Bridge

SPC 'B'SPC 'A'

SEISMIC PERFORMANCE CATEGORY

New York City (Downstate) Area

For bridges in SPC A: V = 0 for a non-critical bridge (Screened out and rated as 5 or 4, Sec. 3.2)

V = V1 for a critical bridge

For bridges in SPC B: V = V1 for a non-critical bridgeV = maximum of V1 or V2 for a critical bridge

For bridges in NYC (Downstate) Area : V = V1 for a non-critical bridge

V = maximum of V1 or V2 for a critical bridge

This process is summarized below.

Figure 3.1 - Assignment of Bridge Vulnerability Score (V)

Revised 11/2002


The determination of these vulnerability scores requires considerable engineeringjudgment. In order to assist in this process, a methodology is given in Sections 3.3.1.1and 3.3.1.2.

Vulnerability scores (V) may assume any value between 0 and 10. A rating of 0 means avery low likelihood of unacceptable seismic damage, a 5 indicates a moderatevulnerability to collapse or a high vulnerability to loss of access, and a 10 means a highvulnerability to collapse. This should not be interpreted to mean that the vulnerabilityscore must assume one of these three values; intermediate values may be assigned.

A comparison of the above two vulnerability scores,V1 and V2, can be used to obtain anindication of the type of retrofitting needed, especially for critical bridges in SPC B or theNYC (Downstate) Area. If the vulnerability score for the bearings V1 is equal to or lessthan the vulnerability score of other components V2, simple retrofitting of only thebearings may be of little value. Conversely, if the bearing score is greater, then benefitsmay be obtained by retrofitting only the bearings.

3.3.1.1 Vulnerability Score for Connections, Bearings and Seatwidths, V1 - Bearings are usedto transfer loads from the superstructure to the substructure and between superstructuresegments at in-span hinge seats. For the purpose of this discussion, bearings areconsidered to include restraints provided at these locations, including shear keys, restrainer bars, and the like. Bearings may be "fixed" bearings, which do not provide fortranslational movement, or expansion bearings, which do permit such movements, asshown in Figure 3.2. A bearing may provide for translation in one orthogonal directionbut not in the other.

Five basic types of bearings are used in bridge construction. These are:

(1) The rocker bearing, which is generally constructed of steel and permits translationand rotational movement. It is considered to be the most seismically vulnerable of all bridge bearings because it usually has a large vertical dimension, is difficult torestrain, and can become unstable after a limited movement and overturn. It alsofails under transverse loading.

(2) The roller bearing, which is also usually constructed of steel. It is stable during anearthquake, except that it can become misaligned and horizontally displaced. Ithas minimal transverse load resistance.

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FIXED BEARINGS

EXPANSION ROCKER BEARINGS

Figure 3.2 - Typical Seismically Vulnerable Bearings.

(3) The elastomeric bearing pad, which has become popular in recent years. It isconstructed of a natural or synthetic elastomer and may be internally reinforcedwith steel shims. It relies on the distortion of the elastomer to provide formovement. This bearing is generally stable during an earthquake, although it has been known to "walk out" under severe shaking due to inadequate fastening.

“Walking out” is mitigated through the use of an internal shear pin or bearingplates with anchor bolts.

(4) The sliding bearing, in which one surface slides over another and which mayconsist of almost any material from an asbestos sheet between two concretesurfaces to PTFE (teflon and similar materials) and stainless-steel plates. Keeperbars can resist transverse loads when multiple bearings are used.

(5) High-load, multi-rotational bearings such as pot, disc, and spherical bearings. These engineered bearings usually have adequate strength for earthquake loads,but have failed in their connections (i.e. keeper bars and anchor bolts) in pastearthquakes.

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Transverse restraint of the superstructure is almost always provided at the bearings. Common types of restraint are shear keys, keeper bars, or anchor bolts. Restraints areusually not ductile, and are subjected to large seismically induced forces resultingfrom a redistribution of force from ductile components such as columns. In addition,when several individual bearings with keeper bars are present at a support, the keeperbars may not resist load equally because of slight variations in clearances. Therefore,individual keeper bars may be subjected to very high forces. In vulnerable structures,collapse may occur due to loss of support resulting from large relative transverse orlongitudinal movement at the bearings. Table 3.1 describes the types of bearings thatcan or can not be expected to provide adequate transverse resistance. The expectedmovement at a bearing is dependent on many factors and cannot be easily calculated. The NYSDOT Specifications require a minimum support length at all bearings innewly constructed bridges [8]. Because it is very difficult to predict relativemovement, the minimum support lengths, N, as required by the NYSDOTSpecifications, may be used here as the basis for checking the adequacy of longitudinalsupport lengths. The definition and equation for determining N is shown in Appendix“D”.

TABLE 3.1 (Revised 11/2004)

Column ATransverse Restraint Expected

1. Substructure with concrete shear keys.

2. Elastomeric pad with center pin.

3. Elastomeric pad with center pin andanchor bolts.

4. Sliding bearings, or multi-rotationalbearings, with guide bars and 4 ormore girders in section.

Column BTransverse Restraint

Not Expected

1. Rocker bearings.

2. Roller bearings.

3. Elastomeric bearings with bearingplates and anchor bolts.

4. Sliding bearings, or multi-rotationalbearings, without guide bars or with 3or fewer girders in the section.

Support skew has a major effect on the performance of bridge bearings. In thismanual, skew is defined as the angle between the support centerline and a line normalto the bridge centerline. Rocker bearings have been the most vulnerable in pastearthquakes. At highly skewed supports, these bearings may overturn during evenmoderate seismic shaking. In such cases, it is necessary to consider the potential forcollapse of the span, which will depend to a large extent on the geometry of thebearing seat. Settlement and vertical misalignment of a span due to an overturnedbearing may be a minor problem, resulting in only a temporary loss of access whichcan be restored, in many cases, by backfilling with asphalt or other similar material. (The potential for total loss of support should be the primary criteria whenassessing the vulnerability of the bearings.)

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STEP 1

Are bearing details satisfactory ?

STEP 2

No

Yes

No Restraint fails ?

Yes

Yes 2 or 3 girder bridge with outside girder on seat edge ?

No No

High Pedestals ( > 12" avg. height) ?

Yes Yes

Rocker Bearings ?

No

Overturning of Bearings possible ? No

Yes Yes

Bridge Collapse likely ?

No

VT = 0 VT = 5 VT = 10

BA

Check transverse behavior

Figure 3.4a - Calculation of Bearing Vulnerability

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Check longitudinal behavior

STEP 3

N < L

N/2 < L < N

Rocker Bearings ?

Overturning of bearings possible ?

STEP 4

V1 = Maximum of VT , VL

VL = 5

V1 = 0

VL = 0 VL = 10

Figure 3.4b - Calculation of Bearing Vulnerability (Cont'd)

Yes

No

No

Yes

No

Yes

Yes

No

BA

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A suggested step-by-step method for determining the vulnerability score forconnections, bearings, and seats (V1) is detailed in the flow chart of Figure 3.4 and isdescribed below. Note that V1 need not be calculated for non-critical bridges in SPCA. See Section 3.1.1 and Figure 3.1.

Step 1: Determine if the bridge has satisfactory bearing details. These bridgesinclude:

a. Continuous superstructures with integral abutments.b. Continuous superstructures with seat-type abutments where all of the

following conditions are met:

(1) Either (a) the skew is less than 20o, or(b) the skew is greater than 20o but less than 30o and the

length-to-width ratio of the bridge deck is greater than1.5.

(2) Rocker bearings are not used.

(3) The bearing seat under the abutment end-diaphragm is continuousin the transverse direction and the bridge has more than threegirders.

(4) The support length is equal to, or greater than, the minimumrequired support length (N) as defined by the NYSDOT StandardSpecifications for Highway Bridges [8].

If the bearing details are determined to be satisfactory, a vulnerability score V1, of0 may be assigned and the remaining steps for bearings omitted. Bridges withunsatisfactory bearing details are addressed in Steps 2 through 4.

Step 2: Determine the vulnerability to structure collapse or loss of bridge accessdue to transverse movement, VT.

Before significant transverse movement can occur, the transverse restraint mustfail. If this occurs, superstructure girders are vulnerable to loss of support ifeither of the following conditions exist:

a. Individual girders are supported on rocker bearings and individual pedestalsor columns. The pedestals/columns have an average height greater than 300mm (12 inches).

b. The exterior girder in a 2- or 3-girder bridge is supported near the edge of abearing seat (less than 200 mm (8 inches)) regardless of whether the bearingsare on individual pedestals or not.

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In either of these cases, the vulnerability score, VT, should be 10.

Steel rocker bearings have been known to overturn transversely, resulting in apermanent superstructure displacement. These bearings are particularlyvulnerable when the support skew is greater than 300. When bearings arevulnerable to a toppling failure but structure collapse is unlikely as determined bya. and b. above, the vulnerability score, VT, should be 5. If collapse is likely, VTshould be 10.

Step 3: Determine the vulnerability of the structure to collapse or loss of accessdue to excessive longitudinal movement, VL.

If the longitudinal support length measured in a direction perpendicular to thesupport is less than one times, but greater than one-half times, the required longitudinal support length (N), the vulnerability score, VL, shall be assigned avalue of 5. If, in addition, rocker bearings are present and are vulnerable tooverturning, a value of 10 for VL should be used. If the longitudinal supportlength is less than one-half of the required longitudinal support length, then avulnerability score, VL, of 10 should be assigned regardless of bearing type.

Step 4: Calculate vulnerability score for connections, V1, from values VT and VL;i.e., V1 = maximum value of VT or VL.

3.3.1.2 Vulnerability Score for Piers, Abutments, and Liquefaction Potential, V2 - Thevulnerability rating for the other components in the bridge that are susceptible to failure,V2, is calculated from the individual component ratings as follows:

V2 = PV + AV + LV # 10 (3.2)

where PV = Pier vulnerability scoreAV = Abutment vulnerability scoreLV = Liquefaction vulnerability score

Note that V2 need only be calculated for critical bridges in SPC B or the NYC(Downstate) Area. See Section 3.3.1 and Figure 3.1.

Methods for calculating each of these component scores are given in the followingsections.

A. Pier Vulnerability Score, PV - Piers generally add to the seismic vulnerability ofbridges. Each type of pier design behaves uniquely when subjected to seismic loading.

Step 1: Assign a pier vulnerability score, PV, of 0 if bearing keeper bars or anchorbolts can be relied upon to fail (Section 3.3.1.1 and Table 3.1), eliminating the transferof load to the piers.

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Step 2: Masonry or stone piers receive a vulnerability score, PV, of 10.

Step 3: If piers and footings have adequate transverse steel, detailed in as-built plans, as required by the NYSDOT Specifications, assign a pier vulnerability score, PV, of 0.

Step 4: If none of the above apply, use one of the following assessment procedures forthe type of pier in question.

1. Solid Piers - Gravity Type - Generally gravity type piers are of old constructionand are either unreinforced or very lightly reinforced. They may experience severecracking when subjected to seismic loads. Assign a pier vulnerability score, PV,equal to 10.

2. Solid Piers - Cantilever Type - Generally solid cantilever piers have morereinforcement than gravity type piers. They are also influenced by the effects ofskew, superstructure continuity and strength of reinforcement. Scores for thesefactors, R, are shown in Table 3.2. Therefore these types of piers receive a basevulnerability score, BV, equal to 6 and are modified by R as shown below:

PV = BV - R (3.3)

3. Multi-column Piers - Piers with multiple columns act differently than do solidpiers*. Pier columns have failed in past earthquakes due to lack of adequatetransverse reinforcement and/or poor structural details. Excessive ductility demandsfrom seismic loading have resulted in column failure in shear or flexure. In pastearthquakes some columns have failed in shear, resulting in column disintergrationand vertical displacements. Column failure may also occur due to pullout of thelongitudinal reinforcing steel, mainly at the footings. Piers with columns on top of a solid plinth are generally controlled by the column behavior with the effectiveheight of the column being measured from the top of the plinth to the bottom of thecap beam.

Multi column piers with known reinforcement details are assessed using theprocedures in Parts A and B. Piers with unknown reinforcement details are assessedin Part C.

Part A: Column vulnerability due to shear failure.

CV = Q - R (3.4)where Lc Q = 13 - 6 [ ] Ps Fbmax (3.5)

Lc = effective column length.Ps = amount of main reinforcing steel expressed as a percent of the column cross-

sectional area.

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F = framing factor:2 for multi-column bents fixed top and bottom.1 for multi-column bents fixed at one end.1.5 for box girder superstructure with a single-column bent fixed at top

and bottom.1.25 for superstructures other than box girders with a single-column

bent fixed at top and bottom. bmax = maximum transverse column dimension.R = the number of points to be deducted from Q for factors known to reduce

susceptibility to shear failure, as shown in Table 3.2.

Table 3.2. Values for R. (Revised 11/2004)

Factor R

Acceleration coefficient, A < 0.4 3

Skew # 20o 2

Continuous superstructure, integral abutments of equalstiffness and length-to-width ratio < 4

1

Grade 40 (or below) reinforcement 1

Values of CV less than zero or greater than 10 should be assigned values of 0 and10, respectively.

Note that Equation 3.5 was empirically derived based on observations of columnshear failure in bridges during the San Fernando earthquake in 1971. The derivationis given in Appendix B of the 1983 Retrofit Guidelines [6]. This expression hassince been checked against column failures in the Northridge earthquake (1994) andwas found to be a reliable indicator of column damage. However, the columns inthis empirical data set are short-medium height and Equation 3.5 may not apply totall and/or slender columns. In these cases, special studies may be undertaken toestimate Q, R, and CV.

Part B: Pier vulnerability due to flexural failure at column reinforcement splices.

To account for flexural failure at column splices, CV should be set equal to 7 forsingle-column bents supporting super-structures longer than 90 m (300 ft), or forsuperstructures with expansion joints where the column longitudinal reinforcementis spliced at a potential plastic hinge location.

Part C: Where reinforcement details are unknown, assign a Q value equal to 10 forpiers greater than 7m (23 ft.) high, measured from bottom of footing to top of capbeam. Shorter piers, 7m (23ft.) in height and shorter, receive a Q value of 7. Calculate CV using equation 3.4.

Revised11/2002


4. Assign overall pier vulnerability score, PV to the highest value calculated for CV in Parts A, B or C.

B. Abutment Vulnerability Score, AV - Abutment failures during earthquakes do notusually result in total collapse of the bridge. This is especially true for earthquakesof low-to-moderate intensity. Therefore, the abutment vulnerability rating should bebased on damage that would temporarily prevent access to the bridge.

One of the major problems observed in past earthquakes has been the settlement ofapproach fill at the abutment. Large fill settlements are possible in the event ofstructural failures at the abutments due to excessive seismic earth pressures orseismic forces transferred from the superstructure. Certain abutment types, such asspill-through abutments and those without wing walls, may be more vulnerable tothis type of damage than others. Except in unusual cases, the maximum abutmentvulnerability score, AV, will be 5. High unreinforced masonry or laid-up stoneabutments receive an abutment vulnerability score, AV = 7.

For bridges in New York State, AV = 0 unless both of the following conditions aresatisfied, in which case AV = 5. These conditions are:

a. The bridge crosses water, andb. The expected fill settlement is greater than 150 mm (6 inches).

Expected fill settlements for bridges over water in New York State may be estimatedat one percent of the fill height measured from the roadway pavement to the base ofthe embankment. Intermediate values for AV (i.e., between 0 and 5) may beassigned based on the presence of wing walls and the type of abutment.

C. Liquefaction Vulnerability Score, LV - Although there are several possibletypes of ground failure that can result in bridge damage during an earthquake,instability resulting from liquefaction is the most significant. The vulnerabilityrating for foundation soil is therefore based on:

a. A quantitative assessment of liquefaction susceptibility.b. The magnitude of the acceleration coefficient.c. An assessment of the susceptibility of the bridge structure itself to damage

resulting from liquefaction-induced ground movement.

The vulnerability of different types of bridge structures to liquefaction has beenillustrated by failures during past earthquakes. Observed damage has confirmed thatbridges with continuous superstructures and supports can withstand largetranslational displacements and usually remain serviceable with minor repairs. However, bridges with discontinuous super-structures and/or non-ductile supportingmembers are usually severely damaged as a result of liquefaction. Theseobservations have been taken into account in developing the vulnerability scoringprocedure described below. The procedure is based on the following steps:Step 1: Determine the susceptibility of foundation soils to liquefaction.

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High susceptibility is associated with the following conditions:

a. Where the foundation soil providing lateral support to piles or vertical supportto footings comprise, on average, saturated loose sands, saturated silty sands,or non-plastic silts.

b. Where similar soils underlie abutment fills or are present as continuous seams,which could lead to abutment slope failures.

Moderate susceptibility is associated with saturated foundation soils that are, onaverage, saturated medium dense soils; e.g., compact sands.

Low susceptibility is associated with foundation soils that are, on average, densesoils.

Step 2: Use Table 3.3 to determine the potential for liquefaction-related damagewhere susceptible soil conditions exist.

Table 3.3. Potential for Liquefaction-Related Damage.

SoilSusceptibility

toLiquefaction

Potential for liquifaction-related damage.

lowmoderate

high

lowlow

moderate

Step 3: Bridges subjected to moderate liquefaction-related damage shall be assigned avulnerability rating, LV, of 5. This rating should be increased to between 6 and 10 if thevulnerability rating for the bearings, V1, is greater than or equal to 5.

Step 4: Bridges subjected to low liquefaction-related damage shall be assigned avulnerability rating, LV, of 0.

3.3.2 Seismic Hazard Score (E) - In this procedure, seismic hazard is a function of the seismicperformance category (SPC) and the site coefficient which allows for soil amplificationeffects. The seismic hazard score is therefore defined as follows:

For bridges in SPC A: E = 1.1 S (3.5 a)

For bridges in SPC B: E = 2.4 S (3.5 b)

For bridges in NYC (Downstate) Area: E = 2.4 S (3.5 c)

where S = site coefficient*.

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It will be seen that E ranges from 1.1 (SPC ‘A’, S = 1) to 8.4 (NYC Area, S = Fv = 3.5) in NewYork State.

For SPC ‘A’ and SPC ‘B’ bridges:

When the soil profile can be determined with confidence, Table 3.4 should be used toobtain the site coefficients. If there is insufficient data for this purpose, Table 3.5 may beused to obtain the site coefficients.

Table 3.4. Site Coefficient, S.

Soil Profile Type Site Coefficient

IIIIIIIV

1.01.21.52.0

Soil Profiles are defined as follows:

Soil Profile Type IA soil profile composed of rock of any description, either shale-like or crystalline innature, or of stiff soils where the soil depth is less than 60 m (200 ft) and the soilsoverlying rock are stable deposits of sands, gravels, or stiff clays, shall be taken as TypeI.

Soil Profile Type IIA soil profile with stiff cohesive or deep cohesionless soil where the soil depth exceeds60 m (200 ft) and the soil overlying the rock are stable deposits of sands, gravels, or stiffclays, shall be taken as Type II.

Soil Profile Type IIIA soil profile with soft to medium-stiff clays and sands, characterized by 9 m (30 ft) ormore of soft to medium-stiff clays with or without intervening layers of sand or othercohesionless soils, shall be taken as Type III.

Soil Profile Type IVA soil profile with soft clays or silts greater than 12 m (40 ft) in depth shall be taken asType IV.

*Site Coefficient, S, and Soil Profile Types from Table 3.4 or Table 3.5 may be used for SPC‘A’ and SPC ‘B’ bridges only. For NYC (Downstate) Area bridges, see Section 6B andAppendix to Section 6B-2 in Division I-A of the NYSDOT Standard Specifications for HighwayBridges [8].Site Coefficients Fa (Table 1.4.2.3a) or Fv (Table 1.4.2.3b), for Soil Profile Types A - E , may besubstituted for Site Coefficient, S, in Equation 3.5c.

Revised11/2002


Table 3.5. Alternative Site Coefficients, S1

Site2 Soil Profile Site Coefficient

land or water crossing

land crossing

water crossing3

water crossing3

rock

all soils

all soils exceptdeep deposits of soft

clay or silt4

deep deposits of soft clayor silt4

1.0

1.2

1.5

2.0

Notes: 1. This table of site coefficients may be used when soil properties are not known in sufficient detail todetermine the soil profile types used in Table 3.4.

2. If a bridge crosses both water and land, the requirements for water crossings shall govern.3. Water crossings include marshes and wetlands.4. "Deep" deposits are those that exceed 12m (40 ft) in thickness.

For NYC (Downstate) Bridges:

See Section 6B and Appendix to Section 6B-2 in Division I-A of the NYSDOT StandardSpecifications for Highway Bridges [8]. Site Coefficients Fa (Table 1.4.2.3a) or Fv (Table1.4.2.3b), for Soil Profile Types A - E , may be substituted for Site Coefficient, S, inEquation 3.5c.

3.4 Assignment of Seismic Vulnerability Class - A seismic vulnerability class is assignedto each bridge based on the classification score calculated in Section 3.3, and inaccordance with ranges defined in Table 3.6.

Table 3.6. Seismic Vulnerability Classes

Classification Score, CS Vulnerability Class

> 7025 - 75

< 30

HighMedium

Low

Overlapping ranges are used to provide the evaluator with some discretion in assigning avulnerability class. For bridges in New York State, the maximum value for theClassification Score is 84 (V = 10, E = 8.4). Bridges that are determined to have High or Medium Seismic Vulnerability are progressed to the Rating Step (Section 4) ahead ofthose with Low Vulnerability. These bridges might also be recommended for interimseismic retrofitting, should they be judged to be particularly vulnerable, and theconsequences of failure are clearly unacceptable.

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SECTION 4 - RATINGPage 4.1

FunctionalClassification

TrafficVolume

LikelihoodScore

ConsequenceScore

Exposure Score

VulnerabilityRating

BridgeClassification

Failure Type Score

SECTION 4 - RATING

4.1 General - The Vulnerability Rating process is common to all six identified BSA failuremodes and it is intended to provide a uniform measure of a structure's vulnerability tofailure on the basis of the likelihood of a failure occurring and the consequences of afailure.

There are six possible vulnerability ratings as shown in Table 4.1. The six ratingsindicate the type of corrective actions needed to reduce the failure vulnerability of abridge and the urgency in which these actions should be implemented. Definitions arefound in Appendix C.

Figure 4.1 shows an overview of the rating process and a detailed description is found inSection 4.2. Bridges may be rated without the use of this manual, however completedocumentation justifying the rating must be submitted to the Structures Division.

Table 4.1 Vulnerability Rating Descriptions

RATING DESCRIPTION

123456

Safety Program WatchSafety Program AlertCapital ProgramInspection ProgramNo ActionNot Applicable

Figure 4.1 - Vulnerability Rating ProcedureRevised 11/2002


4.2 Rating Procedures - The vulnerability rating process is similar to the classifyingprocess, in that scores are assigned to evaluate the likelihood and consequence of afailure and then these rating scores are combined, as shown in Equation (4.1), todetermine the vulnerability rating score.

Vulnerability = Likelihood + Consequence (4.1)Rating Score Score Score

The vulnerability rating (1 through 6) is determined using the rating score ranges shownin Table 4.2. Overlapping ranges are provided to allow the evaluator some discretion inchoosing the appropriate rating. A rating outside the recommended ranges may be used,however complete documentation must be submitted to the Structures Division.

Table 4.2 Vulnerability rating score ranges

Rating ScoringRange

123456

> 1513 - 16

9 - 14 < 15 < 9 ---

The likelihood and consequence scores are weighted equally in the rating equation. Thelikelihood score is determined using the results of the classifying process and theconsequence score is determined on the basis of the type of failure which is anticipatedand the public exposure to that failure.

Figure 4.2 can be used as a worksheet for completing the ratings and as a summary sheetfor the results. Detailed descriptions of the criteria for evaluating the likelihood andconsequence of a failure are found in Sections 4.2.1 and 4.2.2 respectively.

Bridges which are not vulnerable to a particular failure mode should be rated 6, for thatmode. For instance, bridges not over water are not vulnerable to hydraulic failures, andsimilarly, concrete bridges are not vulnerable to the steel detail failures. In theseinstances the vulnerability rating score can be disregarded and a rating of 6 assigned tothe structure.


4.2.1 Likelihood of a Failure - The likelihood of failure score is determined using the resultsof the classifying process. If available, the results of a detailed engineering analysis mayalso be used to supplement the results of the classifying process. Table 4.3 providesscores which should be assigned to the different vulnerability categories.

The vulnerability classes (High, Medium and Low) are the same as previously defined inSections 3.1 and 3.4 of the classifying step. If there is no vulnerability to a particularfailure mode the Vulnerability Rating Score shall be zero. The likelihood scoredetermined from Table 4.3 should be used in Equation (4.1) to determine thevulnerability rating score.

Table 4.3 Likelihood of failure scores

VulnerabilityClass

LikelihoodScore

HighMediumLowNot Vulnerable

10620

4.2.2 Consequence of Failure - The consequence of failure is evaluated on the basis of thetype of failure the bridge is prone to and the exposure to the public that a failure wouldcause. The result of this evaluation will be a consequence score determined as shown inEquation (4.2). This score is used in Equation (4.1) to determine the vulnerability ratingscore.

Consequence = Failure Type + Exposure (4.2)Score Score Score

Descriptions of the failure type and exposure criteria evaluation procedures follow.

a. Failure Type - Failure type is a measure of the way in which a bridge fails. Whenevaluating this parameter, the actual vulnerability of a bridge to the specific failuremode is not considered and it is assumed that a failure has or will take place. Thetask of the rating engineer is to decide what the failure would look like. That is, willit be a sudden and complete collapse with potentially catastrophic consequences orwill it be a partial or localized failure that may or may not affect the serviceability ofthe structure.

Three failure types have been defined and are shown in Table 4.4.


Failures due to seismic forces generally will involve movement of the substructures,such as tilting of a pier or settlement of an abutment, which results in a loss of supportor shifting of the superstructure. Shear and flexural failures in the substructures isanother possible failure mode. To evaluate the type of failure a bridge is prone to,both the superstructure and the substructure configurations must be considered. Forexample, a simply supported, multigirder bridge is prone to catastrophic failure caused by large relative movements at the expansion joints and loss of support due toinsufficient seat widths. On the other hand, a continuous multigirder bridge isunlikely to collapse in this way, but may suffer damage to the piers due to highershear forces.

Table 4.4 Failure type definitions

Catastrophic - The structure is vulnerable to a sudden and complete collapse of a superstructure span or spans. Thisfailure may be the result of a partial or total failure of either the superstructure or the substructure. A failure of thistype would endanger the lives of those on or under the structure.

Partial Collapse - The structure is vulnerable to major deformation or discontinuities of a span (which would resultin loss of service to traffic on or under the bridge). This failure may be the result of tipping or tilting of thesubstructure causing deformations in the superstructure. A failure of this type may endanger the lives of some ofthose crossing or under the structure.

Structural Damage - The structure is vulnerable to localized failures. This failure may be the result of excessivedeformation or cracking in the primary superstructure or substructure members of the bridge. A failure of this typemay be unnoticed by the traveling public but would require repair once it is discovered.

In some instances it may be necessary to obtain additional assistance from experts inother fields, such as geotechnical engineers.

Some factors which should be considered to evaluate the failure type are listed below. Combinations of these and other factors will determine the potential failure type of astructure.

! Redundancy of the Superstructure (internally and externally)! Simple span vs Continuous spans! Bridge type! Span length! Support conditions! Abutments and Piers:

Type SizeHeight FoundationsBearing types Seat widths

Rating scores are assigned for the different failure types, as shown in Table 4.5. These scores are used in Equation (4.2) to determine the consequence of failurescore.


Table 4.5 Failure type rating scores

Failure Type Score

CatastrophicPartial Collapse

Structural Damage

531

b. Exposure - The exposure parameter is a measure of the affect that a failure of a structurewill have on the users of the bridge and the highway network. The exposure score isdetermined on the basis of the traffic volume on the bridge and the functionalclassification of the highway carried by the bridge. The score is determined as shown inEquation (4.3). This score is used in Equation (4.2) to determine the consequence score.

Exposure = Traffic Volume + Functional Classification (4.3)Score Score Score

Rating scores for traffic and functional classification are assigned as shown in Table 4.6. These scores are used in Equation (4.3).

Table 4.6 Exposure rating scores

Traffic Volume Functional Classification

AADT ScoreFunctional

Classification Score

> 25,0004,000 - 25,000< 4,000

210

Interstate & FreewayArterialCollectorLocal Road & Below

3210

The functional classifications are based on the definitions listed in the BIIS manual [1]for the feature carried by the structure.


DATE______________ RC ______ BIN ______________ NAME_______________________ CARRIED ____________________________ CROSSED______________________________

LIKELIHOOD SCORE:Vulnerability ClassHigh = 10Medium = 6Low = 2Not Vulnerable = 0 ________________

CONSEQUENCE SCORE:Failure TypeCatastrophic = 5Partial Collapse = 3Structural Damage = 1 ________________

EXPOSURE SCORE:Traffic Volume> 25,000 AADT = 24,000 - 25,000 AADT = 1< 4,000 AADT = 0 ________________

Functional Classification ScoreInterstate & Freeway = 3Arterial = 2Collector = 1Local Road & Below = 0 ________________

TOTAL = ______________________________________________________________________________________________

VULNERABILITY RATING :Scoring Range Rating > 15 1 13 - 16 2 9 - 14 3

< 15 4 < 9 5

N / A 6 ________________

Figure 4.2 - Vulnerability Rating Summary Sheet

SEISMIC VULNERABILITY MANUAL REFERENCESPage 5.1

SECTION 5 - REFERENCES

1. "Bridge Inventory and Inspection Systems", Manual, New York State Department ofTransportation, Structures Design and Construction Division, 1990, and Amendments 1991,220 pp.

2. "Seismic Retrofitting Manual for Highway Bridges", Federal Highway Administration,Report FHWA-RD-94-052, 1995, 309 pp.

3. "A Policy on Bridge Safety Assurance", New York State Department of Transportation,1992.

4. "Seismic Design References", California State Department of Transportation, Division ofStructures, 1990.

5. "Seismic Design", Standard Specifications for Highway Bridges, Division I-A, 15th Ed,American Association State Highway and Transportation Officials, 1996, and Interims 1997,1998.

6. "Seismic Retrofitting Guidelines for Highway Bridges, Federal Highway AdministrationReport, FHWA/RD-83/007, 1983.

7. “Uniform Code of Bridge Inspection”, NYS Department of Transportation, October 1989

8. New York State Standard Specifications for Highway Bridges, NYS Department ofTransportation, June 1999.

Revised 11/2002

SEISMIC VULNERABILITY MANUAL APPENDIXPage A.1

APPENDIX A

FUNCTIONAL IMPORTANCE (BRIDGE CRITICALITY)

SPC ‘A” and SPC ‘B’ Bridges:

For guidance in determining Bridge Criticality, please refer to Article 6A.4 in Division 1Aof

the NYSDOT Standard Specifications for Highway Bridges [8].

NYC (Downstate) Area Bridges:

For the NYC (Downstate) Area, a “critical” bridge is defined in Table 6B.3-1 in Division 1Aof the NYSDOT Standard Specifications for Highway Bridges [8]. Additional requirementsfor critical bridges are included in Article 6B.3 in Division 1A of the NYSDOT StandardSpecifications.

Revised 11/2002

SEISMIC VULNERABILITY MANUAL APPENDIXPage B.1

APPENDIX B

NEW YORK STATE SEISMIC PERFORMANCE CATEGORIES

SPC ‘A’ SPC ‘B’Region 2 - All Counties Region 1 - All CountiesRegion 3 - All Counties Region 4

Region 4 (Genesee, Orleans, Monroe(Wayne, Ontario and and Wyoming Counties) Livingston Counties) Region 5Region 5 (Erie and Niagara Counties)(Cattaraugus and Chautauqua Region 7Counties) (St. Lawrence, FranklinRegion 6 - All Counties and Clinton Counties)Region 7 Region 8(Jefferson and Lewis Counties) (Columbia, Dutchess, Putnam,Region 9 Orange and Ulster Counties)(Broome, Chenango, Otsego Region 9and Delaware Counties) (Schoharie and Sullivan

Counties) Region 10 (Suffolk County)

NEW YORK CITY (DOWNSTATE) AREARegion 8(Rockland and Westchester Counties)Region 10(Nassau County)Region 11 - All Counties

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SEISMIC VULNERABILITY MANUALSEISMIC VULNERABILITY MANUAL APPENDIXPage B.2

Revised 11/2002

SEISMIC VULNERABILITY MANUAL APPENDIXPage C.1

APPENDIX C

VULNERABILITY RATING SCALE

1. SAFETY PROGRAM WATCH - This rating designates a vulnerability to failure resultingfrom loads or events that may occur in the next few years. Corrective or mitigating action,enhanced inspection or other appropriate safety action, such as placing on a flood watch,shall be taken. If corrective or mitigating action is not immediately taken, placing the bridgeon the current 5-Year Capital Program along with appropriate interim safety action such ascontinued monitoring or traffic restrictions shall be considered.

2. SAFETY PROGRAM ALERT - This rating designates a vulnerability to failure resultingfrom loads or events that may occur, but are not likely in the next few years. Remedial workto reduce the vulnerability or enhanced monitoring is not an immediate priority, but may beneeded in the near future. Placing the bridge on the Capital Program should be considered.

3. CAPITAL PROGRAM ACTION - This rating designates a vulnerability to failure resultingfrom extreme loads or events that are possible but not likely. This risk can be tolerated untila normal capital construction project can be implemented.

4. INSPECTION PROGRAM ACTION - This rating designates a vulnerability to failurepresenting minimal risk providing that anticipated conditions or loads on the structure do notchange. Unexpected failure can be avoided during the remaining life of the structure byperforming the normal scheduled bridge inspections with attention to factors influencing thevulnerability of the structure.

5. NO ACTION - This rating designates a vulnerability to failure which is less than or equal tothe vulnerability of a structure built to the current design standards. Likelihood of failure isremote.

6. NOT APPLICABLE - This rating designates there is no exposure to a specific type ofvulnerability.

Revised 112002

SEISMIC VULNERABILITY MANUAL APPENDIXPage D.1

APPENDIX D

Minimum Support Length Requirements for Seismic Performance Categories A & B(Note: Use for NYC Downstate Area bridges where A # 0.19. For A > 0.19, see Page D.2)

Minimum support length (N) in the longitudinal direction should be measuredperpendicular from the end of the centerline of the girder/beam to the edge of the bridgeseat. Minimum support length (N) in the transverse direction should be measuredperpendicular to the centerline of the girder/beam.

N = ( 8 + 0.02L + 0.08H ) ( 1 + 0.000125S 2 ) ( inches) Equation Aor

N = ( 203 + 1.67L + 6.66H ) ( 1 + 0.000125S 2 ) ( mm ) Equation B

where

L = length of continuous bridge deck, in feet for Eq. A or meters for Eq. B

S = angle of skew of support in degrees, measured from a line normal to the span.

and H is given by one of the following:

for abutments, H is the average height, in feet for Eq. A or meters for Eq. B, of columns supporting the bridge deck to the next expansion joint. H = 0 for single span bridges.

for columns and or piers, H is the column or pier height, from the top of footing to the top of pier/pedestal, in feet for Eq. A or meters for Eq. B .

for hinges within a span, H is the average height of the adjacent two columns or piers in feet for Eq. A or meters for Eq. B .

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SEISMIC VULNERABILITY MANUALSEISMIC VULNERABILITY MANUAL APPENDIXPage D.2

Minimum Support Length Requirements for Seismic Performance Categories C & D(Note: Use for NYC Downstate Area bridges where A > 0.19)

Minimum support length (N) in the longitudinal direction should be measuredperpendicular from the end of the centerline of the girder/beam to the edge of the bridgeseat. Minimum support length (N) in the transverse direction should be measuredperpendicular to the centerline of the girder/beam.

N = ( 12 + 0.03L + 0.12H ) ( 1 + 0.000125S 2 ) ( inches) Equation Cor

N = ( 305 + 2.5L + 10H ) ( 1 + 0.000125S 2 ) ( mm ) Equation D

where

L = length of continuous bridge deck, in feet for Eq. C or meters for Eq. D

S = angle of skew of support in degrees, measured from a line normal to the span.

and H is given by one of the following:

for abutments, H is the average height, in feet for Eq. C or meters for Eq. D, of columns supporting the bridge deck to the next expansion joint. H = 0 for single span bridges.

for columns and or piers, H is the column or pier height, from the top of footing to the top of pier/pedestal, in feet for Eq. C or meters for Eq. D .

for hinges within a span, H is the average height of the adjacent two columns or piers in feet for Eq. C or meters for Eq. D .

Revised 11/2004

Documents

SEISMIC VULNERABILITY MANUALof bridges as to the extent of their vulnerability to these failure modes. The procedure that follows clearly outlines the NYSDOT approach to the seismic